The Importance of Urban Trees in Stormwater Management

May 21, 2015

Drainage problems arising from increasing levels of urbanization have exacerbated the limitations of conventional surface-water drainage measures. Street trees can be essential components to the management of stormwater in urban areas. Like their woodland forest equivalents, urban trees direct precipitation into the ground through trunk flow and absorb rainfall through their roots – making them an invaluable sustainability asset in the urban environment.

Traditional drainage systems for surface water runoff have been designed to transfer rainwater from where it has fallen to either a soak-away or a watercourse as rapidly as possible. This method increases the risks of flooding, environmental damage, and urban diffuse pollution; since runoff water usually carries contaminants including oils, heavy metals, pesticides, fertilizers, chemicals and other urban matter. LID, SUDS, and WSUD are all methods designed to respond to these increasing drainage challenges.

Governmental officials and urban planners are increasingly mandating that developers adopt a sustainable approach to urban drainage. The implementation of sustainable drainage systems – demonstrated in outline as well as in detailed planning applications and design submissions – is now demanded by authorities, from early site evaluations through to the completion of EIAs (Environmental Impact Assessments). Regulations for new buildings have also been modified to introduce a LID protocol, establishing it as part of normal best practice.

One of the key principles of Low Impact Development (LID) is to reduce the negative effects of urban stormwater runoff, including flooding and deteriorating water quality. Street trees are known to significantly reduce stormwater runoff volumes from urban catchments (McPherson, 2004).

Trees also take up less space than other “green systems” such as bio-filtration systems, rain gardens, and vegetative swales; so therefore using street trees as a LID principle helps with space constraints faced by cities. And as part of their natural growth cycles, urban trees have been shown to significantly reduce rainwater nitrogen and other pollution loads in stormwater runoff (Denman, 2007). In addition, tree roots also penetrate through typically impermeable urban soil layers into more permeable zones, thus have the potential to further increase stormwater infiltration rates (Day and Dickenson, 2008). And with the application of appropriate root management, urban landscape designers can ensure that tree root systems only spread to intended areas and avoid paved surfaces.

Due to the many benefits of street trees, they have always been a vital component of the urban landscape. But it’s only recently that trees have been considered an important stormwater management measure. Now that urban trees are finally being recognized by authorities as an effective means of managing stormwater, urban planners and landscape professionals can take the ongoing steps of integrating them into the urban development process.

A proven method to improve street tree growth and mitigate urban stormwater runoff is the implementation of engineered tree pits that increase the availability of quality soil in order to promote optimal root growth and increase runoff infiltration. Urban tree planting systems such as the GreenBlue ArborSystem™, provide everything that street trees need to thrive in urban environments.

The ArborFlow™ sustainable urban drainage tree pit system is about combining stormwater management with urban tree planting design. The drainage system has been developed as an effective and environmentally robust means of managing stormwater runoff. It significantly reduces the velocity and flow rate of surface water runoff in urban areas and contributes towards meeting the required discharge rates. Our stormwater management tree pit system also filters out harmful pollutants using a special species of reed that collects, processes and breaks down pollutants and contaminants carried in surface water. The system prevents particles in the water from passing through, or clogging up the troughs. ArborFlow’s design allows the water to be discharged into surrounding subsoil to be absorbed by the trees’ root system, or to find its way into the specially designed flow-control chamber positioned on the outfall of the tree pit.

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